WASHINGTON (CNN) --Intelligence reports gathered over the past several weeks point to possible maritime attacks in the Red Sea -- including plans to fly airplanes into U.S. and coalition warships, U.S. government sources told CNN on Thursday.

The reports, sources said, were a key reason the U.S. wanted to keep secret the fact it had top al Qaeda operative Abd al-Rahim al-Nashiri in custody.

The secret was part of an effort to ensure other al Qaeda members would not become aware of the type of intelligence the United States had.

Officials described the reports as "credible" but "uncorroborated" regarding the airplane attacks but that they were taking the reports seriously.

Besides threats of airplane attacks on U.S. and coalition warships, there were also threats of attacks on commercial shipping, even before the recent attack on the French oil tanker Limburg.

Al-Nashiri is described as being responsible for planning the "concept of operations" for al Qaeda maritime attacks from the Strait of Gibraltar in the Mediterranean to the Strait of Malacca in the Far East.

He also was said to have been intensively involved in al Qaeda operations in Yemen, including planning for the attack on the USS Cole and the failed attack on the USS The Sullivans.

Officials could not confirm that there also had been threats of attacks on U.S. 5th Fleet headquarters in Bahrain, but noted in recent weeks the threat warning conditions had on more than one occasion been raised to "Delta"-- the highest threat level.

According to the military, threat condition Delta is usually declared as a localized warning when a terrorist attack has occurred or intelligence indicates likely terrorist action against a specific location.

It requires commanders to implement mandatory security measures, and authorizes and encourages them to supplement the mandatory measures as they see fit.